There exist many satellite communication systems that provide communication and networking services to defense, public, and commercial users. However, many of the existing communication systems rely on large satellites, where energy availability for message transmission is not a critical constraint.
With advances in technology, low-cost smaller satellites are finding new applications for defense, public and commercial services. These smaller satellites are typically low earth orbiting satellites (orbital altitude generally between 100 and 1200 miles) and may be particularly useful in providing network connectivity in remote and hard-to-reach areas on Earth. These satellites may also serve as message “ferries” to relay messages between ground stations and user devices in remote locations that have limited satellite coverage.
In addition to their function or mission, satellites are often characterized by size or more specifically with respect to their mass. Various categories of satellites ranging from large satellites having a mass in excess of 1,000 kilograms to femto satellites having a mass less than 0.1 kilogram are currently recognized today by the industry. For purposes of explanation only the remainder of this specification will utilize the term nanosat or nanosatellite to illustrate the teachings of this innovation. However, it should be noted the innovation is equally applicable to all other small satellite categories including Mini, Micro, Pico and Femto satellites, as well as medium and large mass satellites.
As each nanosat passes over ground stations and remote user devices, decisions need to be made as to which message(s) to deliver. Since nanosats orbit close to the earth, they have short contact time windows with a particular ground station or a remote user device. In addition, depending on the orientations of the satellites with respect to the sun, they may have short charging time windows between message deliveries at two different destinations during which their solar panels can be charged to provide power for relaying messages. Also, due to their size, nanosats have battery capacity limitations. Thus, with nanosatellites, energy management is a critical issue for message delivery scheduling.
Various scheduling schemes for satellites and ground stations have been proposed, some of which take into account energy management, and some of which do not. Each of these schemes addresses a unique aspect of scheduling based on a particular application.
For example, in one approach, multiple communication tasks from ground stations to multiple satellites are scheduled with the objective of maximizing the total weight of task priorities to meet available satellite resources (number of transponders, bandwidth, power, support for different service types, etc.). According to yet another approach, a communication task represents a point-to-point connection between a ground station and a satellite, and the communication task is characterized by its bandwidth demand, power need, time requirement, service type and priority. A similar scheduling scheme minimizes the number of unscheduled tasks. These approaches do not consider satellite energy storage capacity or battery charging dynamics.
Another approach schedules downloads from imaging satellites to a multi-ground station network with the objective of maximizing the total amount of data downloaded under energy and data dynamical constraints. Although this approach considers energy constraints, the approach is intended for imaging and data collection applications.
In all the approaches mentioned above, scheduling is performed by a centralized ground control system, in which operators pre-calculate a sequence of messages to be delivered, resulting in static data collection and delivery schedules. For routine messages that are planned, a centralized scheduling method may be satisfactory. However, for nanosats, which must store messages received from remote devices or ground stations and wait until a nanosat-to-ground contact time window with the intended recipient to deliver the message, a centralized scheme adds delay and transmission overheads and reduces flexibility.
A dynamic programming approach has been proposed to address energy limitations for optimizing energy allocation and admission control for communications satellites. According to this approach, a threshold-based optimal policy is derived for a single energy-constrained satellite to select which requests to serve. While this approach does not rely on a centralized ground control system, it does not address the difficulties involved in a nanosat delivering messages to different destinations during nanosat-to-ground contact time windows.
In view of the above, there is a need for a dynamic, decentralized onboard nanosat message delivery scheme in which each nanosat becomes a decision maker and determines its own message delivery scheduling policy, taking into account energy constraints.